MoPub, an ad serving startup for smartphone apps, is announcing a new way for its publishers to offer their inventory to advertisers — a private marketplace limited to select publishers and advertisers.

Basically, the market creates a more direct relationship, where publishers get more control and predictable pricing, while advertisers get early access. Advertisers will get first look a publisher’s inventory — MoPub compares the marketplace to a eBay’s Buy It Now model, where buyers can skip the auction process and just purchase an item at a set price (in this case, an ad impression). They also get access to special data like demographics, geography, and in-app purchase history.

When I spoke CEO Jim Payne last year, he compared MoPub’s ad serving technology, which helps publishers manage a mix of advertising from a number of sources, to desktop ad companies like DoubleClick and AdMeld, but dealing with the specific issues of mobile. Similarly, Payne says the private market is similar to marketplaces for desktop ad impressions, but it’s dealing with the specific data points that are interesting to mobile advertisers, like latitude/longitude coordinates. Among MobPub customers, Payne predicts the marketplace this will be of most interest to “large publishers with high-quality inventory.”

The company has been testing the marketplace in private beta mode, but now it’s opening it up to more publishers and advertisers. The next step, Payne says, is to make it more sophisticated about what data gets passed to advertisers.